The goal of these investigations is to study two of the molecular events that lead to progression of human melanoma with the emphasis in developing therapeutic means to block metastases formation. The molecular changes associated with the transition of melanoma cells from radial growth phase (RGP) to vertical growth phase (VGP) are not very well defined. The expression of the glycoprotein MUC18 in melanoma is directly correlated with increasing tumor thickness of biopsies and with the metastatic potential of human melanoma cell lines in nude mice. In addition, increasing experimental evidence has demonstrated that the progression of human cutaneous melanoma is associated with loss of expression of c-KIT receptor and that the ligand for c-KIT, the stem cell factor (SCF) inhibits the growth of melanoma cell lines expressing c-KIT in vitro. Based on these observations, it seems that gaining of MUC18 and loss of c-KIT expression which occur at the RGP to VGP transition may play a pivotal role in the metastasis of human melanoma. The specific aims of the proposal are four: The first objective is to provide direct evidence for the involvement of MUC18 to melanoma metastasis by transfecting low metastatic and MUC18 negative cells with the MUC18 gene and subsequently to analyze their metastatic potential in vivo. The current goals are to use anti-MUC18 antibody or antisense MUC18 RNA expression vectors to block tumor growth and metastasis in vivo. In their second aim, an in depth analysis will be performed to determine the transcriptional control of the MUC18 gene. These studies may provide future intervention to block MUC18 expression. The third aim is to analyze the role of c-KIT in the progression of human melanoma. Transfection of highly metastatic melanoma cells (c-KIT negative) with the c-KIT gene, has already been shown to decrease their tumorigenicity and suppress their metastatic potential in vivo. In the present proposal, the plan is to investigate the mechanisms by which SCF regulates the growth of human melanoma cells expressing c-KIT in vitro and in vivo, including the hypothesis that melanoma cells expressing c- KIT might regress through the induction of apoptosis by SCF. SCF/c-KIT may also regulate the expression of several other genes involved in cell- cycle progression and invasiveness. The ultimate goal, however, (specific aim four), is to use a packaging cell line that produces retroviruses containing the c-KIT gene for cancer therapy and treatment of established metastases in mice.